The kinetics of mononuclear phagocyte proliferation from various murine sources are being studied in detail to determine the precursor cells' proliferative capacity, their cell cycle parameters and growth fraction. The role of macrophage growth factor and serum factors in the regulation of proliferation are being defined. Since we can culture individual colony forming cells the functional heterogeneity of these cells can be explicitly defined to determine whether heterogeneity of function is associated with distinct subpopulations or whether every macrophage, regardless of source, can express every function. Local environmental factors, e.g. lymphokines, which influence macrophage function are also being investigated.